Production of metals



Aug. 14, 1962 R. ALLEN ET AL 3,049,421

PRODUCTION OF METALS Filed Aug. 27, 1958 Vacuum q Pump Power pp y lnerf Gas Vacuum Pump INVENTORS' All BYCl uxrkr A Qaek VIM/ P J. clW A 3,049,421 Patented Aug. 14, 1962 3,049,421 PRQDUCTEON F METALS Lloyd R. Allen, Belmont, Charles A. Baer, Needham, and

Philip J. Clough, Reading, Mass, assignors to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Filed Aug. 27, 1958, Ser. No. 757,537 11 Claims. (Cl. 75-.5)

This invention relates to the production of metals and more particularly to the production of extremely fine metal powders.

A principal object of the present invention is to provide an economical, simple, vacuum evaporation process for the production of metal powders.

Another object of the invention is to provide a process of the above type for the production of high purity metal powders having a particle size of less than about 0.1 micron.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the product possessing the features, properties, and the relation of com.- ponents and the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing which is a diagrammatic, schematic view of one embodiment of the invention.

Some of the many known ways of producing metal powders comprise (a) the hydrogen reduction of certain readily reducible metal oxides, carbonates, nitrates or formates, (b) the alkali metal reduction of certain metal halides, (0) metal amalgamation and subsequent removal of mercury by distillation, (d) metal hydride decomposition, (e) carbonyl decomposition, (f) halide decomposition, (g) electrolysis and (h) arc disintegration. By the present invention, it is possible to conduct a simple, economical, commercially feasible process not heretofore achieved for the production of high purity metal powders having a particle size as small as or smaller than that produced by any one of the above processes.

The numerous uses of fine metal powders are well known. For example, fine metal powders have been used as catalysts, in pigment and powder metallurgical applications and where they possess pyrophoric properties as fuels in explosives, missiles and the like. In the present process the pyrophoric metal powders produced are of a very high purity having a uniform particle size of less than 0.1 micron. Since these ultrafine metal powders are substantially oxygen-free and have such tremendous surface areas, they possess unexcelled burning characteristics which make them especially useful as rocket and missile fuels. Additionally, because the pure metals produced according to the present process are uniformly smaller than the wavelength of the visible spectrum, they are jet black and thus perfect heat absorbers. Therefore, they find additional usefulness where excellent heat absorption is required or desired.

The present process comprises the thermal evaporation of a metal selected from the group consisting of aluminum, manganese, silver, chromium, beryllium, copper, boron, silicon, iron, nickel, zinc, magnesium, titanium, zirconium, thorium and bismuth at a pressure below about 500 microns and the condensation in free space, under similar pressure conditions, of the metal vapors produced. This condensation, in the absence of oxidizing conditions, gives a resultant product consisting of oxidefree, black, spherical, metal powders having an extremely high purity and a particle size distribution such that substantially all of the particles have a diameter of less than about 0.1 micron. The metal powders are then collected in a non-oxidizing or inert medium. In one embodiment of the invention, the inert medium comprises an organic material such as hexane, heptane, paraffin and the like which will form an oxidation-inhibiting environment for the metal powders. In another embodiment, the metal powders are collected in an inert-gas-filled container which is then sealed against leakage of oxygen or nitrogen. For metal powders to be used as catalysts, a gas such as hydrogen might be adsorbed thereon before sealing.

The invention will be more fully described in the following non-limiting examples.

Example I An induction coil with a graphite crucible therein and suitably insulated therefrom was secured in a vacuumtight tank such that a large free space existed between the crucible and the tank wall opposite the crucible. Aluminum and a few chips of zirconium to promote wetting were placed in the crucible. Also provided within the tanks were means for feeding additional quantities of aluminum and zirconium to the crucible.

The tank was closed and evacuated to a pressure on the order of about 30-50 microns to remove most of the residual gases. During the evacuation the induction heating coil was energized and the aluminum brought up to melting temperatures. During this period, the pressure was adjusted by bleeding in argon. When the desired pressure of 150 microns had been obtained, the aluminum melt temperature was raised to about 1250 C. so as to cause evaporation of the aluminum.

To maintain the melt level, aluminum was periodically fed by lowering a segmented rod of aluminum into the crucible. The rod was segmented by lateral saw-cuts every few inches to reduce the thermal conductivity up the rod. The saw-cuts served to provide a measure of aluminum being added and speeded up the melting of the segment inserted in the crucible due to the poor heat transfer up the rod. Zirconium chips were inserted in the saw-cuts and fed to the crucible in order to minimize the formation of aluminum carbide on the surface of the melt.

Visual inspection of the aluminum vapor stream showed that it condensed in the free space above the crucible as a billowing cloud and that the condensed metal collected at the bottom of the tank. The height of the condensing vapor column was a function of the operating pressure; increasing in height and becoming more diffuse as the operating pressure decreased. Upon completion of the aluminum evaporation and condensation in free space, the powder was collected in a container in the bottom of the tank. The tank and container were then flooded with nitrogen and the container was sealed, with an atmosphere of nitrogen therein. The container was removed from the vacuum chamber and the container was filled with liquid heptane so that all of the powder was covered by the heptane.

The aluminum powder produced was pyrophoric, jet black in color and had a dry bulk density of 0.08 gram per cc. (fluffy) and 0.22 gram per cc. (tamped). Electron microscope photographs at 11,900 magnification showed the aluminum powder to be spherical in shape with a particle size within the range of 0.011 to 0.06 micron to 600' Angstrom units). The average particle size was approximately 0.03 micron.

Example 11 Nickel powder was produced in manner similar to that described in Example I with the exception that the evapocondense as billowing cloud in free space.

ration wa carried out at a temperature above about 1500 C. and at a pressure of between about 100 and 200 microns. Visual inspection of the nickel vapor stream showed it to be condensing as a billowing cloud in the free space, above the crucible and settling to the bottom of the tank. The nickel powder produced was in all appearances identical to the aluminum powder obtained in Example I.

In addition to aluminum and nickel, high purity, ultrafine powders of manganese, silver, chromium, beryllium, copper, boron, silicon, iron, zinc, magnesium, bismuth, titanium, thorium, zirconium and other metals and metalloids can also be produced by thermal evaporation and condensation in free space at pressures below about 500 microns.

The metal to be converted into powder can be melted and evaporated by suitable heating means such as induction heaters, resistance heaters, electron bombardment and the like. One preferred method is to evaporate the metal from a suitably heated source having a large effective metal evaporation area and containing therein a molten pool of metal. Evaporation of the metal in this way avoids excessive splattering and favors the formation of more uniform size particles. The achievement of uniformity of such small particle sizes has heretofore not been possible.

The temperatures required for evaporating the metals, of course, depends upon the vapor pressure of the particular metal and the operating pressures employed. The temperature at which the evaporation is carried out, determines the rate of metal vapors effiux from the source containing the molten metal. Temperature at which the vapor pressure of the metal is below approximately 0.1 millimeter of mercury will yield low evaporation rates while higher temperatures and correspondingly higher metal vapor pressure will yield higher evaporation rates. The rates of evaporation which can be employed are quite broad and can be varied considerably.

The pressures employed for the metal evaporation and condensation of the metal vapor are below about 500 microns and preferably between about and 200 microns. The pressures employed can be obtained by evacuating the system to an extremely low pressure and then adjusting to a higher pressure with an inert medium such as argon, helium and the like. The height of the vapor stream emanating from the metal vapor source is largely dependent on the pressure employed. At the low pressures the Vapor stream is long. As the operating pressure rises the length of the vapor stream decreases. Thus when operating at any of the preferred pressures, it is essential that the free space between the metal vapor source and the surface opposite it be sutficiently large so that the metal vapors condense in the free space and not on the surface opposite the metal vapor source. In the practice of the invention, it is preferable to operate at pressures which produce metal vapor streams which Additionally, by operating at the pressures indicated metal powders substantially free of impurities such as oxygen are obtained.

The high purity metal powders produced have a uniform particle size of less than about 0.1 micron and generally less than about 0.06 micron. The metal powders have an enormous surface area and, for the most part, are so very readily oxidized, when exposed to air, that ignition occurs. When the high purity of the metal powders is to be retained, then the collection thereof protect the powders from oxidation. Combinations of the various inert mediums described can be used too. For instance, the powder can be collected and screened under vacuum conditions and then stored and shipped in containers in a non-reactive organic liquid or solid.

As mentioned, the preferred method of handling the metal powders consists in collecting the powders in a liquid, organic material. After production the powders are wet down with a suitable liquid, organic material and handled as a wet cake or the powders can be processed as such and then collected in containers under a liquid, organic material of choice, e.g. hexane, heptane, benzene, naphtha and the like. The powders can also be collected and stored in a normally solid, organic material such a parafiin by maintaining the wax in the liquid phase during collection and then permitting it to solidify. In the wet state or where imbedded in an organic solid the powder is not pyrophoric and the bulk density is raised to a point where it can be inexpensively shipped. Oxygen contamination is also prevented.

The organic material to be used should possess certain properties. For instance, it should be non-reactive with the powders and should protect the powders from spontaneous ignition or oxidation. It should also be such that, if desired, it can be subsequently removed without affecting the agglomerate particle size or properties of the powders. It is obvious that there are many cases where the organic material selected is one which can be used with the powder in its applications or in the later processing of the powder so that it need not be removed.

Clearly, the number of organic materials which can be employed is enormous and the properties and suitability of any one can be readily determined from the standard texts.

In applications where only the small particle size is desired and the purity is of no significance, then the ultrafine powders can be produced using a controlling oxidizing atmosphere or by careful exposure to air while closely controlling the temperature of the powders. Temperature control in this latter instance is critical since, if the temperature rises too high, then spontaneous ignition occurs. The temperature is controlled until the necessary oxidation reaction is essentially completed. Aluminum powders treated in this way have a very thin oxide film (about 25 Angstroms) which protects the remainder of the powder sphere.

One preferred type of equipment for producing the metal powders in accordance with the invention is shown in the drawing wherein 10 represents a vacuum-tight tank or chamber which is evacuated through conduit 12 by means of suitable pumping system. The lower portion of tank 10 is preferably funnel shaped for facilitating the collection and discharge of the metal powders from the tank. Within tank 10 there is a vapor source 14, here shown as crucible means for holding a charge of the metal such as aluminum to be melted and evaporated. The vapor source 14 is suitably heated by means 16 illustrated as an induction heating means. The distance between the vapor source 14 and the tank wall opposite vapor source 14 should be such that even at very low pressures the metal vapors must condense in free space and not on the tank wall. Obviously, more than one vapor source and other types of vapor sources and heating means than those shown can be employed. Suitable means such as vibrators for moving powders collecting on the Walls of the tank can also be provided. Likewise tank 10 can also be provided with external cooling means if desired.

If continuous operations are desired, then suitable means (illustrated as a vacuum-tight seal 32) for feeding additional quantities of metal 18 to the vapor source 14 can be provided. The additional metal can be fed to the crucible in rod form as shown, as a ribbon, wire, pellet, powder, or even as a liquid.

Valve means 20 is provided between the bottom of tank 10 and powder collecting chamber 22. A non-oxidizing medium is provided within chamber 22. This is preferably accomplished by evacuating chamber 22 through conduit 24 to a low pressure by means of a suitable vacuum pumping system. A source of inert gas can be provided for filling chamber 22 with any desired amount of such inert gas.

Valve means 26 is provided between collecting chamber 22 and storage or shipping receiver 28. A non-oxidizing medium is created within receiver 28 utilizing conduit 30 in the same manner as conduit 24. Suitable screening means, not shown, can be located between chamber 22 and receiver 28. If such screening means are employed then a plurality of receivers can be suitably coupled to collect the various sized metal powders.

In continuous operations not only may a charge of metal be placed in source or sources 14, but means for feeding additional quantities of metal are also provided. Additional metal is fed to the source at a rate to replenish that which has evaporated and to maintain a substantially constant melt level. The metal vapors condensing in free space settle to the lower portion of tank 10. Periodically, valve 20 is opened (valve 26 being closed) to permit an appreciable quantity of metal powder to fall into chamber 22 which is preferably maintained at a reduced pressure. Valve 26 is then opened (valve 20 being closed) to permit the powder to fall into receiver 28. Receiver 28 is either filled with an inert, organic liquid or inert gas or maintained under a reduced pressure. Screening of the metal powders as produced can be done at any point. Valve 26 is then closed and chamber 22 is then prepared for the next batch of powder. The receiver 28 is sealed and removed. A new receiver is placed into position and prepared for use.

Since certain changes can be made in the above without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process for the production of high purity, ultrafine metal powders which comprises thermally evaporating a metal selected from the group consisting of aluminum, manganese, silver, chromium, beryllium, copper, boron, silicon, iron, nickel, zinc, magnesium, bismuth,

titanium, zirconium and thorium at a pressure below about 500 microns Hg abs., condensing the resultant metal vapors in free space, said free space being maintained at a pressure below about 500 microns, and collecting the resultant spherical, high purity powders of a uniform particle size of less than about 0.1 micron in an inert medium.

2. The process of claim 1 wherein the evaporation of metal and condensation of metal vapor in free space is carried out at a pressure between about and 200 microns Hg abs.

3. The process for the production of high purity, ultrafine aluminum powders which comprises thermally evaporating aluminum at a temperature above about 1000 C., and at a pressure below about 500 microns Hg abs., condensing the resultant aluminum vapor in free space, said free space being maintained at a pressure below about 500 microns Hg abs., and collecting the resultant spherical, high purity powders of a uniform particle size of less than about 0.1 micron in an inert medium.

4. The process for the production of ultrafine metal powders which comprises thermally evaporating in a system maintained at a pressure below about 500 microns Hg abs. a metal selected from the group consisting of aluminum, manganese, silver, chromium, beryllium, copper, boron, silicon, iron, nickel, zinc, magnesium, bismuth, titanium, zirconium and thorium, and condensing the vapors in free space, said free space being maintained at a pressure below about 500 microns Hg abs., and recovering the spherical powders of a uniform particle size of less than about 0.1 micron in an inert medium from the system.

5. The process for the production of high purity ultrafine metal powders which comprises thermally evaporating a metal selected from the group consisting of aluminum, manganese, silver, chromium, beryllium, copper, boron, silicon, iron, nickel, zinc, magnesium, bismuth, titanium, zirconium and thorium at a pressure below about 500 microns Hg abs., condensing the resultant metal vapors in free space, said free space being maintained at a pressure below about 500 microns Hg abs., and thereafter without subjecting to oxidizing conditions, collecting the resultant spherical, high purity powders of a uniform particle size of less than about 0.1 micron in a nonoxidizing inert medium.

6. The process of claim 5 wherein the non-oxidizing inert medium comprises a vacuum.

7. The process of claim 5 wherein the non-oxidizing inert medium comprises an inert gas.

8. The process of claim 5 wherein the non-oxidizing inert medium comprises an organic coating on said powders.

9. The process of claim 5 wherein the non-oxidizing inert medium comprises an organic liquid.

10. The process of claim 5 wherein the non-oxidizing inert medium comprises a molten organic material which is normally solid at room temperature.

11. Black aluminum powder consisting of spherical particles essentially free of surface oxidation, the powder having a particle size distribution such that substantially all of the particles have diameters less than .1 micron, the powder being pyrophoric and being immersed in a protective non-oxidizing medium.

References Cited in the file of this patent UNITED STATES PATENTS 1,524,101 Newell Jan. 27, 1925 1,574,988 Marx Mar. 2, 1926 1,584,728 Case May 18, 1926 1,594,345 Bakken et al Aug. 3, 1926 1,762,716 Grine June 10, 1930 2,051,863 Kemmer Aug. 25, 1936 2,437,815 Hansgirg Mar. 16, 1948 2,584,660 Bancroft Feb. 5, 1952 2,828,199 Findlay Mar. 25, 1958 OTHER REFERENCES Carman: Metallurgia, vol. 52, Oct. 1955, pp. -168. Published by the Kennedy Press, Ltd., Manchester, England.

Kopelman: Electrical Engineering, May 1952, pp. 447- 451. Published by the American Institute of Electrical Engineers, New York, NY.

Frazier: Proceedings of the 14th Annual Meeting of the Metal Powder Association, April 1958, pp. 65-70, Published by the Metal Powder Association, New York, NY.

Griest, In: Dissertation Abstracts, vol. XV, No. 9, Oct. 1955, page 1588. Published by University Microfilms, Ann Arbor, Mich. 

1. THE PROCESS FOR THE PRODUCTION OF HIGH PURITY, ULTRAFINE METAL POWDERS WHICH COMPRISES THERMALLY EVAPORATING A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, MANGANESE, SILVER, CHROMIUM, BERYLLIUM, COPPER, BORON, SILICON, IRON, NICKEL, ZINC, MANGESIUM, BISMUTH, TITANIUM, ZIRCONIUM AND THORIUM AT A PRESSURE BELOW ABOUT 500 MICRONS HG ABS., CONDENSING THE RESULTANT METAL VAPORS IN FREE SPACE, SAID FREE SPACE BEING MAINTAINED AT A PRESSURE BELOW ABOUT 500 MICRONS, AND COLLECTING THE RESULTANT SPHERICAL, HIGH PURITY POWDERS OF A UNIFORM PARTICLES SIZE OF LESS THAN ABOUT 0.1 MICRON IN AN INERT MEDIUM.
 11. BLACK ALUMINUM POWDER CONSISTING OF SPHERICAL PARTICLES ESSENTIALLY FREE OF SURFACE OXIDATION, THE POWDER HAVING A PARTICLE SIZE DISTRIBUTION SUCH THAT SUBSTANTIALLY ALL OF THE PARTICLES HAVE DIAMETERS LESS THAN 1 MICRON, THE POWDER BEING PYROPHORIC AND BEING IMMERSED IN A PROCTECTIVE NON-OXIDIZING MEDIUM. 